The present invention relates to an impact energy absorbing mechanism applicable to a steering device of a motor vehicle for absorbing impact energy ascribable to the operator's body which is thrown out onto a steering wheel of the steering device in the event of collision of the vehicle. More particularly, the present invention relates to an impact energy absorbing mechanism of the type including an absorbing plate which is bent generally in the form of letter U and, in the event of a collision, caused to deform with its bent portion sequentially shifted so as to absorb impact energy.
The steering device of a modern motor vehicle is usually provided with an impact energy absorbing mechanism to reduce an impact which is applied to a steering wheel when the operator's body hits against the steering wheel due to inertia in the event of collision of the vehicle. A prerequisite with such a mechanism is that the amount of energy absorption for a predetermined stroke of the steering wheel be constant to insure a desired energy absorbing effect.
An impact energy absorbing mechanism for the above-described application has previously been proposed in various forms. One of prior art mechanisms includes an absorbing plate which is bent in the form of a letter U at the time of assembly. Specifically, the generally U-shaped absorbing plate is deformable with its bent portion sequentially shifted to absorb impact energy. This kind of mechanism needs a minimum of exclusive space and, despite a simple structure, achieves a large stroke and therefore a large amount of energy absorption.
Typical of prior art mechanisms of the type using an absorbing plate is disclosed in Japanese Patent Publication No. 46-35526. In this Patent Publication, a steering shaft is divided into an upper shaft and a lower shaft which are telescopically coupled together. One of the upper and lower shafts is provided with a lengthwise channel, and an absorbing plate is received in the channel in such a manner as to form a bend. The opposite ends of the absorbing plate are each fixed to a respective one of the two shafts. When the upper shaft is displaced relative to the lower shaft, the absorbing plate is deformed with its bend sequentially shifted to absorb impact energy.
A problem with the above-described type of prior art mechanism is that it is difficult to surely maintain the radius of curvature of the bend of the absorbing plate constant while the deformation of the plate is under way, because the plate is simply received in a space between the upper and lower shafts. Hence, a constant energy absorbing effect is hard to achieve with such a prior art mechanism. The lengthwise channel formed through the steering shaft is undesirable because it reduces the rigidity of the shaft and thereby prevents sufficient rigidity necessary for the transmission of a steering effort through the shaft from being attained. The decrease in rigidity has to be compensated for by increasing the diameter of the shaft. In addition, an extra machining step is needed to form the lengthwise channel through the shaft. Furthermore, the available stroke of the upper shaft and therefore the energy absorbing ability of the mechanism is limited because the absorbing plate is connected to the upper and lower shafts at opposite ends thereof.
In light of the above, Japanese utility Model Publication No. 690-7262 discloses a mechanism wherein flanges are securely connected to opposite sides of a steering column which rotatably supports a steering shaft. An absorbing plate produced by bending a sheet metal in the form of letter U is provided with recesses which are open toward the steering wheel. The flanges and the absorbing plate are fastened to a vehicle body by bolts with the flanges being interposed between the opposite legs of the absorbing plate. When the operator's body is thrown out onto the steering wheel due to collision, the steering column is moved forward to causes the flanges to sequentially shift the bend portion of the absorbing plate with their edges, thereby absorbing the impact energy.
With the flanges an absorbing plate scheme described above, the absorbing plate is deformed by the edges of the flanges while maintaining the radius of curvature of its bent portion constant and, hence, the amount of energy absorption due to bending deformation remains constant. The impact energy is also absorbed by the friction acting between the absorbing plate and the flanges and vehicle body and the tension or stretching force applied to the absorbing plate. Therefore, the total energy absorbing effect attainable with such a prior art mechanism is considerable. Further, it is not necessary to increase the diameter of the steering shaft because a channel for accommodating the absorbing plate is not needed.
However, the mechanism relying on flanges and an absorbing plate as stated above has a problem left unsolved. Specifically, since the absorbing plate is fastened to a vehicle body by bolts together with the flanges of the steering column, the frictional force acting between the flanges and the vehicle body and contributing to the absorption of impact energy is effected by the fastening force of the bolts. Then, the energy absorption characteristic itself is changed by the fastening force of the bolts, preventing a predetermined energy absorbing effect from being achieved. since it is extremely difficult to maintain the fastening force of the bolts constant and since the coefficient of friction of a member which forms a part of a vehicle body depends upon the kind of motor vehicle, mounting the impact energy absorbing mechanism while insuring a desired energy absorbing characteristic is extremely difficult.